There is quite a number of the phase control type of dimmers in the prior art mainly for controlling the brightness of incandescent and fluorescent lamps. In the majority of these systems the TRIAC has been the device of choice mainly because of its simplicity to be easily configured with any load mostly in line voltage AC applications.
There are also TRIAC related dimmer circuits in the prior art for low voltage AC lighting systems mainly for dimming incandescent lamps. This type of dimming is often desired, for example, in landscaping to achieve the objectives of a particular architectural lighting scheme. Low AC voltages are usually obtained from standard step-down transformers or other low voltage sources such as alternators used in automobiles. An AC line voltage in the primary of a step down transformer can be reduced to 12 volts or 24 volts AC in the secondary. These low voltage sources can deliver high currents that are actually required, for example, in low voltage lighting to obtain high levels of lamp illumination.
A TRIAC based dimmer circuit, with the TRIAC requiring a high trigger voltage of about 35 volts, cannot be used to drive a load directly in a low voltage dimming system. In a number of these systems, in the prior art, the TRIAC is used to drive the primary side of a transformer. This can cause problems due to high voltage spikes and current surges that can be induced in a system when a fast switching device like a TRIAC is driving an inductive load such as the primary of a transformer. As a result of these problems the transformer would become hot and most likely it would be destroyed. The transformer is also at risk of being damaged from the DC voltage which is introduced in the primary because of the asymmetrical triggering of the TRIAC. Another problem with these dimming systems is that they can only drive one type of a transformer either a magnetic or an electronic and in most of these cases a third neutral wire would be needed. Still another problem is that lamps cannot be dimmed individually. Furthermore there are no claims for commercially available dimmers or in prior art that these systems can be used outdoors in conjunction with items such as landscape transformers or light fixtures.
TRIAC's and SCR's can also work in dimmer circuits as static switches. The drawback with these circuits is that they are very temperature dependent and if used outdoors the output voltage would change from where it is set initially. As a result of that the brightness of the lamp would also change. Another problem with these circuits would be the starting of the incandescent lamp becoming unreliable when operating with trigger angles close to 90 degrees. However this type of circuits, by also being able to function while connected in the low voltage side of a transformer or any other low voltage source, would constitute the prior art for the present invention. Changes in circuit performance, as a result of temperature variations, are compensated by means of zener diodes. This would insure that the performance of the circuit remains compensated as long as the circuit is operating at a suitable trigger angle within the trigger angle range for these circuits being from zero degrees to 90 degrees. In this mode of operation the life of the incandescent bulb is extended and a form of soft-start is introduced. Also the starting of the incandescent lamp load becomes very reliable even if operating with trigger angles close to 90 degrees. Furthermore in this type of circuits semiconductor AC devices, such as TRIAC's or SCR's, are driving incandescent lamp loads directly. Hence damaging voltage spikes or current surges do not exist. Also a residual DC voltage in the AC voltage output, due to the asymmetrical triggering of the TRIAC, would cause no problems but if correction is needed it can easily be achieved.